These facts throw a great light on the significance of the structure of the protoplasmic corpuscle which we call a “cell,” and show that the universal presence of the nucleus in every “cell” is due to the fact that it plays the most important part in the life of the cell. It is the seat of control, and contains substances in virtue of which the changes which constitute growth and form-production take place, and in the absence of which the rest of the protoplasm cannot “carry on,” although for a time it lives; that is to say, remains chemically undecomposed, and shows active movement. At the same time, we must not underrate the importance of the general protoplasm, without the presence of some of which the nucleus cannot do its work, nor even exist. It is no wonder, then, that when a cell divides, there are curious and elaborate proceedings in the nucleus, by which each daughter cell gets its due half of the all-important nuclear substance. [Illustration: FIG. 42.—Six successive stages in the division of a “cell,” to show the appearance of the V-shaped filaments of colourable matter or chromatin. _a_, resting-cell, with chromatin dispersed as fine irregular filaments in the nucleus; _b_, the chromatin takes the form of a wreath with twelve loops—it lies horizontally across the cell; _c_, the loops break from one another, and form twelve separate V-shaped pieces; _d_, each of the twelve pieces divides along its length into two parallel V-shaped pieces; _e_, the divided pieces now separate from one another, so as to form two wreath-like groups of twelve V-shaped pieces at each end of the cell; _f_, the wall of the cell forms across between the separated groups of V-shaped pieces, which lose their regular arrangement. Each group becomes enclosed in a capsule, and is the nucleus of a new cell. This is the regular process of cell-division, and the mode in which the chromatin of the nucleus is broken up, so as to be equally shared by the two daughter cells. In some species of animals the cells have as many as thirty-six V-shaped chromatin bodies; in others as few as two. Different plants also show a similar difference in the number of chromatin bodies characteristic of the species.]